The present invention relates generally to the manufacture of semiconductor devices, and more particularly to manufacture of contacts and/or vias that include conductive plugs.
Continuing advances in semiconductor manufacturing processes have resulted in semiconductor devices with finer features and/or higher degrees of integration. Among the various features that may be included within a semiconductor device are contact structures (including xe2x80x9cviasxe2x80x9d) that typically provide an electrical connection between circuit devices and/or layers. The above-mentioned advances have led to contact structures with smaller sizes and/or higher aspect ratios. A contact aspect ratio may be the ratio between a contact depth and width.
A typical contact structure may include forming a contact hole in an insulating layer and then filling such a contact hole. Contact structures with smaller contact sizes and/or higher aspect ratios can be more difficult to fill than larger contacts and/or contacts with lower aspect ratios. Consequently, a contact filling material is often selected for its ability to adequately fill a contact hole.
Two common conductive materials that may be included in a semiconductor manufacturing process are aluminum and copper. Such materials have been included in interconnect patterns and the like. However, it has been difficult to form small and/or high aspect ratio contacts with aluminum. Similarly, while can copper provides advantageously low resistance, it is believed that many technical problems may have to be overcome before copper contact structures may be practically implemented. In view of the above drawbacks to materials such as aluminum and copper, many conventional contact forming methods include tungsten as a contact filling material.
One method of forming contact structures with tungsten includes a selective tungsten chemical vapor deposition (W-CVD) method. In a selective W-CVD method, tungsten may be deposited essentially only on silicon exposed at the bottom of a contact hole. It is believed that current conventional selective W-CVD methods are not sufficiently reproducible to provide satisfactory results in a manufacturing process. Further, adverse results may result when selective W-CVD methods are used to fill contacts having depths that vary. More particularly, a contact hole that is shallow with respect to the other contact holes may suffer from excessive growth (overgrowth) of tungsten in the contact hole. Overgrowth of tungsten may then be corrected with an etch back step that removes only overgrown portions. However, such an etch back step can add to the complexity and/or cost of a manufacturing process.
In light of the drawbacks present in selective W-CVD approaches, conventional xe2x80x9cblanketxe2x80x9d W-CVD methods are widely used for filling contact holes. In a blanket W-CVD method, contact holes may be formed in an insulating layer. Tungsten may then be deposited over the surface of the insulating layer, filling the contact holes. Deposited tungsten may then be etched back to remove tungsten from the top surface of the insulating layer while tungsten within the contact holes remains. Tungsten remaining within a contact hole is often referred to as a tungsten xe2x80x9cplug.xe2x80x9d
A conventional method for forming a tungsten plug in a contact with a blanket W-CVD method will now be described with reference to FIGS. 3A-3D and 4A-4B.
In a conventional contact formation process, an interlayer insulating film 002 may be formed on a silicon substrate 001 that includes an impurity diffusion region 011. An interlayer insulating film 002 may include silicon dioxide (SiO2), for example. A contact hole 020 may then be formed through the interlayer insulating film 002 to the impurity diffusion region 011. A structure following the formation of such a contact hole 020 is shown in FIG. 3A.
Referring now to FIG. 3B, a titanium film 003 may be deposited on the surface of the interlayer insulating film 002, including within the contact hole 020. A titanium film 003 may be deposited with a conventional sputtering method, and to a thickness in the range of about 20 nm to 50 nm. A conventional sputtering method may be isotropic. A titanium film 003 may serve as a barrier layer for subsequent contact materials, preventing such materials from diffusing into a semiconductor substrate 001.
Referring now to FIG. 3C, following the deposition of a titanium film 003, a titanium nitride film 004 may be deposited on the exposed surface, including within the contact hole 020. A titanium nitride film 004 may be deposited with a reactive sputtering method, and to a thickness in the range of about 20 nm to 50 nm. In such a reactive sputtering method, a titanium target may be a source of titanium. Titanium particles from a target may react with nitrogen before reaching a device surface thereby providing titanium nitride as a sputtered material.
A layered film of titanium/titanium nitride (003/004) may serve as an adhesion layer for a subsequently deposited material, such as tungsten. Following the deposition of a layered titanium/titanium nitride film (003/004), a temperature cycling step may be used to further improve the adhering characteristics of such a layered film. As but one example, a ramp anneal may be performed at 650xc2x0 C. for 30 seconds. Such a ramp anneal may result in a reaction between the film materials, as well as a reaction between a titanium film 003 and an interlayer insulating film 002 that furthers the adhering characteristics of the layered film.
Referring now to FIG. 3D, a layer of tungsten 005 may then be deposited over a layered of film of titanium/titanium nitride (003/004). A tungsten deposition step may include a source gas that, includes tungsten, such as tungsten hexafluoride (WF6), as but one example. Such a deposition step may form a layer of tungsten 005 over a layered of film of titanium/titanium nitride (003/004), thereby filling a contact hole 020.
An etch back step may then be performed that removes portions of tungsten on the interlayer insulating film 002 while leaving tungsten within a contact hole 020, thereby forming a tungsten plug. Such a tungsten etch back step may include a fluorine containing gas. For example, tungsten may be plasma etched with sulfur hexafluoride (SF6) as a source gas.
Following the etch back of tungsten, exposed portions of the layered titanium/titanium nitride (003/004) film may be removed with a chlorine containing gas. A contact structure following such a step is shown in FIG. 4A. The result may be a contact structure with a tungsten plug.
Following the formation of a tungsten plug, an interconnect film may be formed over a semiconductor substrate 001, including over a tungsten plug. An interconnect film may include aluminum, as but one example. Such an interconnect film may then be patterned to form an interconnect structure 006. A semiconductor device following the formation of an interconnect structure 006 is shown in FIG. 4B.
In this way, a conventional W-CVD process may be used to form a tungsten plug that connects and interconnect structure 006 to a semiconductor substrate 001.
A drawback to a conventional approach, such as that shown in FIGS. 3A-3D and 4A-4B, can be a resulting shape of a tungsten plug. More particularly, as shown in FIG. 4A, an upper portion tungsten 005 formed within a contact hole 020 may have a recess. Such a recess may be formed when a tungsten film 005 and/or layered titanium/titanium nitride film (003/004) is etched back. More particularly, such layers may essentially be overetched to help ensure that residual tungsten, titanium and/or titanium nitride is not left on a surface of interlayer insulating film 002. Such an overetching can remove an upper portion of tungsten 005 that is within a contact hole 020.
A recess in an upper portion of a tungsten plug (i.e., increased xe2x80x9cplug lossxe2x80x9d), can result in worse step coverage for an overlying interconnect structure 006. FIG. 4B shows such an arrangement. An interconnect structure 006 must extend into a portion of a contact hole 020, over a step formed when a tungsten 005 top surface is lower than an interlayer insulating film 002 top surface. Such a structure may lead to undesirably increased resistance in an interconnect structure 006. Further, in such a structure, material in an interconnect layer 006 may be more susceptible to electromigration.
Plug loss may also present difficulties for subsequent structures. For example, an interconnect structure 006 formed over a tungsten plug having a recess may have an uneven surface. A second interlayer insulating film may be formed over an interconnect structure 006. A via hole may then be etched through the second insulating film to the interconnect structure 006. The uneven surface of an interconnect structure 006 may make it difficult to remove all of a second insulating film. If all of the second insulating film is not removed, a via may have higher contact resistance.
FIG. 10 shows a conventional sputtering apparatus. Such an apparatus may be used to deposit a film of titanium as shown in FIG. 3B. A conventional sputtering apparatus may include a substrate holder 031. A substrate holder 031 can hold a semiconductor substrate 032, that is to be processed, in an essentially parallel orientation to a target 035. A target 035 may be formed from a material that is to be deposited (e.g., titanium).
A magnet 033 may be disposed on one surface of the target 035, while an opposite surface can face a semiconductor substrate 032. A target 035 may also be connected to a DC power source 034.
The application of a voltage to a target 035 can result in sputtering particles 037 being released from the target 035. In the conventional approach illustrated, sputtering particles 037 can be incident on a semiconductor substrate 032 from various directions due to scattering. Consequently, a sputtering apparatus shown in FIG. 10 can provide isotropic sputtering particles.
One approach to addressing plug loss is disclosed in Japanese Laid-Open Patent Publication No. 9-321141. In particular, the publication shows a technique in which the thickness of a titanium nitride layer is thicker than the previously described approach. A titanium nitride layer may have a thickness in the range of 100-200 nm, instead of 20-50 nm. This technique will be explained with reference to FIGS. 5A-5D and 6A-6D.
In the technique of FIGS. 5A-5D and 6A-6D, an interlayer insulating film 002 may be formed on a silicon substrate 001 that includes an impurity diffusion region 011. An interlayer insulating film 002 may include silicon dioxide (SiO2), for example. A contact hole 020 may then be formed through the interlayer insulating film 002 to the impurity diffusion region 011. A structure following the formation of such a contact hole 020 is shown in FIG. 5A.
Referring now to FIG. 5B, a titanium film 003 may be deposited on the surface of the internal insulating film 002, including within the contact hole 020. A titanium film 003 may be deposited with a conventional sputtering method, and to a thickness of about 30 nm. A conventional sputtering method may be isotropic.
Referring now to FIG. 5C, following the deposition of a titanium film 003, a titanium nitride film 004 may be deposited on the exposed surface, including within the contact hole 020. A titanium nitride film 004 may be deposited with a reactive sputtering method, and to a thickness in the range of about 150 nm to 200 nm. A conventional reactive sputtering method may also be isotropic.
Referring now to FIG. 5D, a layer of tungsten 005 may then be deposited over a layered film of titanium/titanium nitride (003/004), thereby filling a contact hole 020.
Referring now to FIG. 6A, an etch back step may then be performed that removes portions of tungsten on the interlayer insulating film 002 until a titanium nitride layer 004 is exposed. Such a tungsten etch back step may include a reactive plasma etch with sulfur hexafluoride (SF6) and argon (Ar) as source gases.
Following the etch back of tungsten, exposed portions of the layered titanium/titanium nitride film (003/004) may be etched. Such an etching may be a two-stage process. In a first step, the layered titanium/titanium nitride film (003/004) may be etched with a reactive ion etch (RIE) having a high selectivity with respect to titanium nitride. Such a RIE step may remove titanium nitride 004 and can expose a titanium layer 003. A structure following such a first step is shown in FIG. 6B.
In a second step, the layered titanium/titanium nitride film (003/004) may be etched with a reactive ion etch (RIE) having a lower reactivity than that of the first step, described above. As but one example, such a second etching step may include a source gas flow rate ratio between chlorine gas (Cl2) and argon gas (Ar) of about 1:30 and a high frequency power of about 450 W. Such a second step may remove portions of the layered titanium/titanium nitride film (003/004) on the surface of a interlayer insulating film 002, thereby forming a tungsten plug, as shown in FIG. 6C.
As in the previously described conventional example, following the formation of a tungsten plug, an interconnect film may be formed over a semiconductor substrate 001, including over a tungsten plug. An interconnect film may include aluminum, as but one example. Such an interconnect film may then be patterned to form an interconnect structure 006. A semiconductor device following the formation of an interconnect structure 006 is shown in FIG. 6D.
In this way, a tungsten plug may be formed that has an upwardly projecting top portion, and not a recess, as is the case of methods that suffer from plug loss.
While the technique of FIGS. 5A-5D and 6A-6D can provide an approach for addressing plug loss, such an approach is not without disadvantages. Such disadvantages will now be described with reference to FIGS. 9A and 9B.
A first disadvantage can be insufficient filling of a contact hole. When a titanium nitride film 004 thickness is increased, the remaining space in a contact hole 020 that is to be filled with tungsten 005 can be significantly reduced. As noted above, a titanium nitride deposition method may be essentially isotropic. Consequently, the thicker titanium nitride film 005 can be formed on the side walls of a contact hole 020. A resulting reduced contact space is shown in FIG. 9A. Such a reduced contact space can be harder to fill by conventional tungsten deposition processes.
Further, an isotropic deposition of titanium nitride can result in an overhanging shape at the upper portion of a contact hole. One example of such an overhanging shape is shown in FIG. 9B. An overhanging shape can reduce the size of the top of a contact hole opening, making it more difficult to subsequently fill the contact hole.
As manufacturing technology continues to advance, contact holes (including via holes) continue to decrease in size. As but one example, contact holes of 0.3 xcexcm or less may be formed. Thus, filling such smaller contact holes in light of the above disadvantage can become an increasingly more difficult task.
A second disadvantage can be an increase in plug resistance. In a technique such as that shown in FIGS. 5A-5D and 6A-6D, a thicker titanium nitride film can be formed on the inner walls of a contact hole. Thus, a contact may include more titanium nitride in cross section than is the case of other conventional methods. Because titanium nitride can have a higher resistance than tungsten, a contact structure according to FIGS. 5A-5D and 6A-6D can have a higher resistance than other conventional approaches.
A third disadvantage can be trenching (or xe2x80x9cgougingxe2x80x9d) on a top portion of a contact structure. Such trenching may occur when titanium nitride is removed by etching. More particularly, when an adhering layer, such as titanium/titanium nitride (003/004) is etched, portions of the adhering layer at the top of a contact structure can be removed, leaving recesses. The formation of such recesses is often referred to as trenching. When adhering layers are relatively thin, such trenching can be relatively small. However, because such a layer is thicker in the method according to FIGS. 5A-5D and 6A-6D, trenching may be large with respect to other conventional approaches. If relatively large trenching occurs, contacts with higher interconnect resistance and/or reduced electromigration resistance may result.
In the method according to FIGS. 5A-5D and 6A-6D, a two step etch method for removing an adhering film may reduce trenching in some cases. However, such a two step approach can add complexity to a manufacturing process. Further, while effective in some cases, such an approach may be less effective in other cases. In a particular, for contact holes having a diameter of 0.3 xcexcm or less, effects of trenching are increased and may not be sufficiently addressed.
In light of the above discussion, it would be desirable to arrive at some way of forming contact structures that can prevent plug loss without incurring the drawbacks of insufficient contact hole filling, increased resistance, or trenching on the top of the contact structure.
According to the present invention, a semiconductor manufacturing process may include forming an insulating film on a semiconductor substrate. A contact hole may then be formed in the first insulating film. A titanium film may then be deposited over the first insulating film and in the contact hole. The titanium film may be deposited with an anisotropic sputtering method to a thickness outside the contact hole of 100 nm or more. A titanium nitride film may then be formed over the titanium film. A tungsten film can then be deposited over the titanium nitride film, including within the contact hole. A first etch step may then remove tungsten to expose the titanium nitride film outside the contact hole. One or more subsequent etch steps may then remove titanium and titanium nitride films outside the contact hole, thereby forming a tungsten plug. An interconnect conductive film may then be formed over the tungsten plug.
According to one aspect of the present invention, by forming the titanium layer with an anisotropic sputtering method, the thickness of the titanium film outside a contact hole may be 100 nm or more, while the thickness of such a film within a contact hole may be substantially smaller. This can enable tungsten to be deposited in the contact hole with fewer defects. Further, when the titanium and titanium nitride films are removed, a tungsten plug may be formed with an upwardly projecting top portion.